September 23, 2009

CARBON CYCLE OF EARTH

The carbon cycle is mainly associated with living matter, although inorganic carbon provides important segments to complete the cycle. The cycling of carbon is strongly controlled by its storage in natural reservoirs. The time period of such storage may range from millennia in rocks, through decades in deep ocean layers to seasons in active biota. Relevant time periods of such storage suggested by Warneck (1988) are:

1. Geological activity involving rocks: 2,400 to 30,000 years

2. Soil humus: 200 years

3. Long-term biosphere storage: 75 years

4. Short-term biosphere storage: 15 years

5. Ocean mixed layers: 4 to 10 years

Estimates of mass content of carbon in various global reservoirs are given in the Table-1.

Carbon in oceans

Major storage of carbon in oceans occurs in the intermediate and deep water below the thermocline. The deep layers of oceans have a very slow mixing period and carbon remains in situ for atleast 20 years in these layers. Far above in oceans, in the mixed layer, which provides the main medium of interchange with the atmosphere, carbon storage is about 1.5 orders of magnitude lower. Ninety percent of the carbon in the oceans is stored as bicarbonate (CO32-) and about 9% as carbonate (CO3–). About 3% of carbon is present in organic matter in environment.

The mixing layer in oceans, broadly the layer above the thermocline, is assumed to be at depth of 75 meters. The average concentration of carbon dioxide in the oceanic surface layer (above the mixing layer) is 2.05 mmol m-3. This concentration rises rapidly with depth to about 2.29 mmol m-3 at the depth of about one-kilometer and remains fairly constant thereafter. The average oceanic carbon dioxide concentration is calculated to be about 2.25 mmol m-3. Since colder ocean water is able to hold more carbon dioxide, variations in its concentration occur with temperature of ocean water. The mass of carbon dioxide in the mixed layer is about the same as that in the atmosphere, with a total exchange between the two estimated to occur over a period of about seven years.

Table-1. Mass content of carbon in global reservoirs

Reservoir

Carbon-content

in Pg (1015 g)

OCEANS

1. Total dissolved CO2

37400.0

2. Dissolved CO2 in mixed layer (75 m depth)

670.0

3. Living biomass carbon

3.0

4. Dissolved organic carbon

1000.0

SEDIMENTS

1. Continental and shelf carbonates

270 x 105

2. Carbonates in oceans

230 x 105

3. Continental & shelf organic carbon

100 x 105

4. Organic carbon in oceans

200 x 104

BIOSPHERE

1. Terrestrial biomass

650.0

2. Soil organic

2000.0

3. Oceanic organic

1000.0

ATMOSPHERE (mostly as CO2)

1. Pre-industrial estimate (290 ppmv)

615.0

2. present estimate (350 ppmv)

734.0

Organic carbon in oceans comes from precipitated remains of living organisms. About 80% of the precipitated material may be redissolved in the deep ocean layers. Dissolved organic carbon content of ocean waters is roughly estimated to be about 0.7 g m-3. Rest of the carbon in the ocean is particulate, mainly as calcium carbonate and this portion of oceanic carbon has a concentration of about 20 mg m-3. Living organisms contribute a total of only 3 Pg to the oceanic carbon storage.

Carbon in sediments and rocks

Carbon makes up only 0.032% of the Earth’s crust by mass. In terrestrial rocks, it is dissolved by rains or surface water over long periods of time and is carried by the surface runoff water to be deposited on the continental shelf sediments. In deeper oceans, deposits from organisms are built up on the ocean floor over millennia. Exchange of carbon from these locations occurs over thousands of years and is associated with activity of Earth’s crust. About two third of this carbon is inorganic carbon and rest is organic in form. Only about 1% of carbon in the form of oil and coal present in Earth’s crust can be used economically. It is estimated that if all the carbon stored in sediments is released suddenly, the atmospheric pressure will rise by 38 bars and the Earth’s atmosphere will become similar to that of planet Venus.

Carbon in biosphere

In the biosphere carbon is exchanged through:

Photosynthetic activity of photosynthetic living organisms, mainly the green plants

Release of carbon on decay and decomposition of dead living organisms

Respiratory activity of all the aerobic living organisms including both plants and animals

Release of carbon from soil humus

The mass of carbon is about three times higher than in living biosphere. The biospheric exchange processes are relatively inactive and the carbon storage may occur for 200 years. Long-lived species, particularly the plants store about 75% of the carbon present in the living biota. The major impacts on global carbon content present in the active biosphere occur in the forests, which store over 80% of the world’s biomass. Though estimates are uncertain because global distribution of different ecosystems is not known accurately, it is quite clear that tropical rain-forests, boreal forests and temperate forests are the most important ecosystems regarding storage and exchange of carbon.

Carbon in atmosphere

Exchange of carbon with the atmosphere occurs mainly through the biosphere with oceanic mixed layer being an important secondary source. Most important atmospheric form of carbon is CO2 gas and global estimates of its exchange between atmosphere and biosphere are:

1. Assimilation of CO2 into plants: 113 Pg Y-1

2. Re-release into atmosphere from:

Respiration of living organisms: 55 Pg Y-1

Microbial decay: 42 Pg Y-1

Soil humus: 10 Pg Y-1

Forest fires and agricultural burning: 1 Pg Y-1

3. Herbivore consumption: 5 Pg Y-1

In addition to CO2, other minor gases in the carbon chain are carbon monoxide (CO), methane (CH4) and non-methane hydrocarbons (NMHCs e.g. HCHO). Carbon dioxide gas is relatively inert while others are quite active in global atmospheric chemistry. Important features of atmospheric carbon species are discussed below.

1. Carbon dioxide: Though CO2 is a minor gas in the atmosphere in comparison with oxygen and nitrogen, it has major impact on global heat balance because of its high capacity of absorbing infra-red radiation. Continuously rising concentration of atmospheric CO2 due to various human activities, particularly the fossil-fuel burning, is major factor in global greenhouse warming. Anthropogenic carbon contributes about 3% of annual carbon loading. Further, its importance in relation to biosphere is supreme since it is required for photosynthesis and existence of biosphere depends on photosynthesis.

2. Carbon monoxide: About 90% of CO originates during photochemical production of methane in atmosphere. Some CO is produced during biomass burning and some during atmospheric oxidation of organic gases that are emitted from vegetation. Highest concentrations of CO are found in middle and high latitudes of Northern Hemisphere, which may reach 150 – 200 ppbv. The concentrations of atmospheric CO show a definite seasonal rhythm and are higher in summers than in winters. In Southern Hemisphere, CO concentrations are lower than in Northern Hemisphere by a factor of upto three. CO is removed from the atmosphere mainly by being oxidized to CO2.

3. Methane: This is a trace gas in atmosphere and is released mainly from rice paddies, wetland areas, enteric fermentation from animals and biomass burning. It has a uniform latitudinal distribution with an average concentration of about 1.6 ppmv. Major sinks of methane are temperate and tropical soils and oxidation to carbon monoxide.

4. NMHCs: This group includes a complex set of hydrocarbons with highly varying characteristics. Most of these are chemically active and have short lifetimes. The usual concentrations in the atmosphere are only few ppbv with localized peaks occurring near the sources. These compounds are removed from atmosphere usually by atmospheric photochemical reactions.

5. Particulate organic carbon (POCs): These complex mixtures of hydrocarbons, alcohols, esters and organics in particulate form. These are usually produced from secondary reactions (gas to particle conversions) and are important in cloud and precipitation processes. The concentrations of POCs in marine air may be around 0.1 to 0.5 g m-3 and in background continental air may be around 1.0 g m-3. In general, the composition of POCs has about 60% neutral compounds, 30% acids and 10% bases.

6. Elemental carbon: This comes into the atmosphere exclusively form biomass and fossil-fuel combustion. Its typical atmospheric concentration over continents is 0.02 g m-3. It is present as fine black powder and can be used as excellent tracer substance for studying long-range transport phenomena in atmosphere.

In addition to above forms, carbon is also present in the atmosphere as carbonyl sulfide, carbon disulfide and dimethyl sulfide. These compounds are important in sulfur-loading of atmosphere and have been discussed with atmospheric sulfur.